import sys

try:
    import mydebug

    def dprint(*objects):
        print(*objects)
except:
    def dprint(*objects, sep='', end='\n'): pass

# d, p = [int(x) for x in input().split()]
# n = int(input())
l, k = [int(x) for x in input().split()]

m = ((l-0.1) / 2) // k 
ans = int(m*k)
print(ans)